The present invention relates to a strengthening member for an automobile used for a front side member, a rear side member, a cross member, a side sill or the like of an automobile.
In the engine room located at the front part of an automobile, front side members are disposed as strengthening members on the left and right sides in the axial direction so as to extend in the fore-and-aft direction of the car body. When a compressive load is exerted on the front side members at the time of head-on collision of a vehicle, the front side members crush in the longitudinal direction, thereby absorbing the energy of the collision. On the other hand, in the event that the front part of an automobile suffers a collision from the lateral direction, the collision energy is absorbed by the bending deformation of a front side member. Likewise, rear side members are disposed at the rear part of an automobile and side sills to a body, as strengthening members.
Such strengthening members for an automobile are required to have the properties of increasing the amount of energy absorption while minimizing the mass per unit length and also being hardly bendable even when an offset load is exerted thereon. As a strengthening member for an automobile having such properties, Japanese Unexamined Patent Publication No. H8-337183 discloses a strengthening member of a lengthy structure obtained by forming steel sheets into a closed section for sustaining a compressive load in the longitudinal axial direction, wherein the closed section is cruciform and has twelve corner portions and four U-shaped portions forming an angle of 90xc2x0 with each other, and each of the U-shaped portions has two side walls and a top wall between the side walls. By forming such a shape, it has been made possible to increase the amount of energy absorption and to obtain a hardly bendable strengthening member without increasing the weight of the member. The strengthening member disclosed in the Publication is composed of two steel sheets divided into left and right portions at the center line of the top and bottom U-shaped portions in the cross section, each of the steel sheets being formed by press forming so as to contain a U-shaped portion in the cross section.
For reducing the weight of a car body while securing collision safety, a strengthening member such as a side member has further been required to reduce the weight (to reduce mass per unit length) and to increase the amount of energy absorption, that is, to improve the crush resistance in the axial direction and to enhance the flexural strength.
In addition, when a high-strength steel sheet is used in forming a strengthening member by a conventional press forming, there has been a problem of being apt to generate cracks at the concave portions of the corners and, as a result, being hardly formable.
An object of the present invention is to provide a lightweight strengthening member for an automobile, excellent in crush resistance in the axial direction and having high flexural strength. Another object of the present invention is to provide a strengthening member which does not generate cracks during forming even when a high-strength steel sheet is used.
The present inventors found that, in a lengthy strengthening member 1 having concave portions 3 at the four corners of a basic rectangular cross section 2 and forming a cruciform closed section as a whole, energy absorption amount could be increased and flexural strength could be enhanced even though the mass per unit length of the member was not increased by making the wall thickness of the corner concave portions 3 thicker than that of the portions excluding the concave portions or by making the hardness of the concave portions 3 harder than that of the portions excluding the concave portions. It is preferable to increase the wall thickness of the concave portions 3 and the hardness thereof at the same time. Further, the present inventors found that the desirable distributions of the above-mentioned wall thickness and hardness could easily be obtained when a strengthening member 1 was formed by drawing or hydroforming.
The present invention has been developed based on the above findings and the gist of the present invention is as follows:
(1) A strengthening member for an automobile, being a lengthy strengthening member 1 having concave portions 3 at the four corners of a basic rectangular cross section 2 and forming a cruciform closed section as a whole, characterized in that the ratio (C/L) of the length (C) of said concave portions 3 to the length (L) of the shorter sides of said basic rectangle 2 is in the range from 0.05 to 0.45 and the strengthening member is formed by drawing or hydroforming.
(2) A strengthening member for an automobile, being a lengthy strengthening member 1 having concave portions 3 at the four corners of a basic rectangular cross section 2 and forming a cruciform closed section as a whole, characterized in that the wall thickness of said concave portions 3 is 5 to 30% thicker than the wall thickness of the portions excluding the concave portions, namely, the side walls 4 and the top walls 5.
(3) A strengthening member for an automobile, being a lengthy strengthening member 1 having concave portions 3 at the four corners of a basic rectangular cross section 2 and forming a cruciform closed section as a whole, characterized in that the hardness of said concave portions 3 is 5% or more higher than the hardness of the portions excluding the concave portions, namely, the side walls 4 and the top walls 5.
(4) A strengthening member for an automobile according to the item (2) or (3), characterized by being formed by drawing.
(5) A strengthening member for an automobile according to the item (1), characterized in that the wall thickness of two sides facing each other in said cruciform closed section is 5 to 30% thicker than the wall thickness of the other two sides facing each other.
(6) A strengthening member for an automobile according to any one of the items (1) to (5), characterized in that the ratio (C/L) of the length (C) of said concave portions 3 to the length (L) of the shorter sides of said basic rectangle 2 is in the range from 0.2 to 0.35.
In a lengthy strengthening member 1 having concave portions 3 at the four corners of a basic rectangular cross section 2 and forming a cruciform closed section as a whole, when said strengthening member is formed by the press forming of steel sheets as described in the above-mentioned Japanese Unexamined Patent Publication No. H8337183, the wall thickness tends to decrease at the concave portions of the corners in the closed section. In addition, it is difficult to enhance the hardness of the concave portions selectively by work hardening. On the other hand, a strengthening member according to the present invention, wherein the wall thickness of the concave portions is made thicker than that of the portions excluding the concave portions or the hardness of the concave portions is made harder than that of the portions excluding the concave portions, can remarkably improve crush resistance in the axial direction and flexural strength compared with a strengthening member described in the above-mentioned Japanese Unexamined Patent Publication No. H8-337183.
In addition, even when a high-strength steel sheet is used for forming a strengthening member for an automobile, such cracks as observed in the case of press working can be avoided by applying drawing or hydroforming as disclosed in the present invention instead of press working, and thus it has become possible to provide a strengthening member for an automobile made of a high-strength steel sheet.